1. Field of the Invention
The present invention relates to a comb-type filter, and more particularly to a feedback comb-type filter used to improve a signal-to-noise (S/N) ratio of a video signal.
2. Description of the Prior Art
In order to improve the signal-to-noise (S/N) ratio of the video signal in video camera, video recorders and the like, the feedback comb-type filter is mainly used.
FIG. 5 is a block diagram of a feedback comb-type filter reported in the "process and picture quality of the video signal by the comb-type filter" in NHK Technical Research Monthly Report, April, 1968. Next, explanation will be given on construction and operation for improving the S/N ratio of the conventional feedback comb-type filter in accordance with FIG. 5.
In FIG. 5, reference numeral 1 designates an input terminal of the video signal, 2 designates an adder whose first input terminal is connected to the input terminal 1 of the video signal, 3 designates a one horizontal time (hereinafter referred to as 1H) delay circuit which is given an output signal of the adder 2 and outputs the inputted signal after delaying of the duration corresponding to 1H, the 1H delay circuit 3 comprising a delay unit 31 and a low pass filter 32 connected in series with each other, and 4 designates an adder whose first terminal is given the output of the 1H delay circuit 3 and whose second terminal is connected to the input terminal 1 of the video signal and given the video signal V.sub.i, the adder 4 introducing its output signal to a signal output terminal 6 and a coefficient multiplier 5, so that the output signal from the adder 4 is introduced to the coefficient multiplier 5 and multiplied therein by a feedback coefficient so as to be given to the second input terminal of the adder 2. Reference numeral 7 designates a level adjusting circuit for introducing therein an output signal of the adder 4 through the signal output terminal 6 and adjusting the inputted signal so as to output it to an adjusted signal output terminal 8.
Such conventional feedback comb-type filter obtains a feedback delay signal by introducing to the coefficient multiplier 5 the output of the circuit and operates as follows:
Now, when a signal level at the video signal input terminal 1 is represented by V.sub.i, an input signal level of the 1H delay circuit 3 by V.sub.H, the signal of the signal output terminal 6 by V.sub.o, the feedback coefficient of the coefficient multiplier 5 by K, and the transfer function of the 1H delay circuit 3 by G, the following equations should hold: EQU V.sub.H =V.sub.i +KV.sub.o ( 1) EQU V.sub.o =V.sub.i +GV.sub.H ( 2)
By eliminating V.sub.H from the equations (1) and (2), the relation between the output signal V.sub.o and the input signal V.sub.i is decided and given in the following equation: ##EQU1## wherein ##EQU2## is the transfer function of the filter constructed as shown in FIG. 5. The transfer function G of the 1H delay circuit 3, when a television signal larger in line correlation is processed, need only be .tau..sub.L =1/f.sub.L (where .tau..sub.L : delay time and f.sub.L : line frequency) and is given in EQU G=e.sup.-J.omega..tau..sbsp.L=cos .omega..tau..sub.L -j sin .omega..tau..sub.L.
Accordingly, the transfer function in the equation (3) is representable in the following equation: ##EQU3## Assuming that n is a plus integer, the equation (4) represents that the output level of the filter shown as FIG. 5 becomes 2/(1-K) times of input signal level in case the frequency f of input signal has the relation such as f=nf.sub.L, and that becomes 0 in case frequency f of input signal has the relation such as f=(n+1/2).multidot.f.sub.L, which indicate that the filter constructed as shown in FIG. 5 is a comb-type filter which produces a loop at a portion of an n multiple in the line frequency. Also, by varying the feedback coefficient K, the sharpness of the amplitude frequency characteristic of the filter changes, which represents that the improvement factor in the S/N ratio can freely be set.
The 1H delay circuit 3 comprises a delay unit 31 and a low pass filter 32 for eliminating needless signals, such as sampling pulse leakage, from the delay unit 31, and is set so that the sum of delay time of both the delay unit 31 and low pass filter 32 becomes 1H.
Also, the level adjusting circuit 7 is provided in order that the signal V.sub.o from the signal output terminal 6, which is the function of the feedback coefficient K, is multiplied by (1-K), thereby not causing variation in the output amplitude, regardless of variation in the feedback coefficient K, in the signal being outputted from the adjusted signal output terminal 8.
The conventional feedback comb-type filter thus changes the feedback coefficient K of the coefficient multiplier 5, thereby changing the improvement factor for S/N ratio, and when the feedback coefficient K is changed, the multiplier (1-K) of the level adjusting circuit as a matter of course must be changed. However, it is difficult to obtain the consistency of the coefficient multiplier 5 and level adjusting circuit 7, whereby it has substantially been difficult to change the feedback coefficient of the coefficient multiplier 5, in other words, to vary the improvement factor for S/N ratio.
When the introduced video signal has no line correlation, a problem of generation of the pseudo signal has been created. Furthermore, a problem has been that another low pass filter other than that for the delay circuit was required for limiting the horizontal frequency band.